The 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as “LTE”) and LTE-Advanced which is an evolved version of LTE (hereinafter, referred to as “LTE-A”) employ orthogonal frequency division multiple access (OFDMA) for the downlink communication scheme.
For frequency scheduling and link adaptation in OFDMA, each terminal (may also be referred to as “UE (User Equipment)”) measures channel information (CSI: Channel State Information) and reports channel information (CSI measurement result) to a base station (may also be referred to as “eNB”). On the other hand, the base station allocates appropriate resources to the terminal using channel information (CSI measurement result).
In LTE, CSI measurement is performed using cell-specific reference signals (CRS). CRS is transmitted in all subframes. The terminal can observe CRS at the time of establishing synchronization.
On the other hand, in LTE-A, CSI measurement is performed using CSI-RS (reference signal for channel quality measurement). It should be noted that since LTE-A is a system which has been extended while maintaining backward compatibility with LTE, CSI-RS which is an LTE-A-specific reference signal is allocated in both time and frequency domains at a low density to minimize insertion loss of CSI-RS. For this reason, the terminal needs to acquire CSI-RS-related parameters as information broadcasted within a cell (broadcast information) by the time CSI-RS allocated at a low density is observed. As CSI-RS-related parameters, the number of transmitting antennas, the positions of time/frequency resources within a subframe and a transmission period and subframe offset or the like are defined.
In LTE, there are two types of operation of CSI measurement and CSI reporting: operation of periodically performing CSI reporting (hereinafter, referred to as “periodic CSI reporting”) and operation of aperiodically performing CSI reporting (“aperiodic CSI reporting”).
In periodic CSI reporting, the terminal performs CSI measurement according to one previously indicated measurement operation out of a plurality of measurement operations in preparation for reporting using defined uplink resources (e.g., uplink control channels), allocates the CSI measurement results to the uplink control channels and reports the CSI measurement results to the base station. An example of measurement operation is CSI reporting assuming closed-loop MIMO (Multiple Input Multiple Output) control. This CSI reporting employs operation of measuring and reporting RI (Rank Indicator) indicating a spatial multiplexing number, a wideband desired precoding matrix (PMI: Precoding Matrix Indicator) and wideband channel quality information (CQI: Channel Quality Indicator). That is, CSI is made up of RI, wideband PMI and wideband CQI.
In aperiodic CSI reporting as well as periodic CSI reporting, the terminal performs CSI measurement according to one previously indicated measurement operation. However, aperiodic CSI reporting is different from periodic CSI reporting in that the terminal reports CSI at timing instructed from the base station and the terminal reports CSI using resources on a common data channel.
As the aforementioned method of previously indicating the measurement operation, a method using a radio resource control message (RRC signaling) is available. Furthermore, aperiodic CSI reporting is instructed through allocation of an uplink data channel (e.g., PUSCH) using a downlink control channel (e.g., PDCCH).
It should be noted that different measurement operations may be indicated in periodic CSI reporting and aperiodic CSI reporting. For example, while RI, wideband PMI and wideband CQI are reported in periodic CSI reporting as described above, RI, wideband PMI and narrow band CQI can also be reported in aperiodic CSI reporting. That is, in aperiodic CSI reporting, narrow band CSI is reported instead of wideband CQI in periodic CSI reporting. In this case, it is possible to use different operations: referencing periodic CSI reporting for rough link adaptation application when transmitting not urgent data and referencing aperiodic CSI reporting for detailed link adaptation application when transmitting urgent data.
In LTE-A, an operation has been added whereby two types of previously indicated measurement targets are respectively reported in periodic CSI reporting. In LTE-A, an operation has been further added whereby two types of previously indicated measurement targets are associated with timings instructed from a base station also in aperiodic CSI reporting, and reporting corresponding to one of the two types of measurement targets is performed at each timing. The two types of measurement targets are indicated using RRC signaling according to a bit-map corresponding to 40 consecutive subframes.
Furthermore, in LTE-A, a concept of carrier aggregation has been introduced, the number of instruction bits for aperiodic CSI reporting has been expanded to 2 bits, and an operation of measuring and reporting two types of previously indicated component carrier groups can be adopted in addition to the operation of measuring and reporting component carriers instructed to be reported.
Moreover, studies on operation of coordinated multiple transmission point (CoMP) are being carried out. CoMP is a technique whereby a plurality of base stations (which may also be referred to as “cell,” “eNB” or “transmission point”) cooperate to transmit signals to a terminal (UE) and several schemes are under study. Examples of main CoMP schemes under study in 3GPP include two schemes: (1) coordinated beam forming (CB) scheme and (2) joint transmission (JT) scheme.
The CB scheme is a scheme whereby data to a certain terminal is possessed by only a specific base station (cell). That is, for the terminal, signals from a cell that possesses no data intended for the terminal (e.g., a cell adjacent to the cell to which the terminal is connected) are regarded as interference. The CB scheme adopts a method of reducing inter-cell interference by controlling transmission parameters. More specifically, examples of transmission parameters include precoding, transmission power, modulation scheme and coding rate. By appropriately controlling these transmission parameters, it is possible to weaken signals from an interference cell (a cell that possesses no data intended for the terminal) for the terminal while strengthening signals from a desired cell (a cell that possesses data intended for the terminal).
On the other hand, the JT scheme is a scheme in which data to a certain terminal is shared by a plurality of base stations (cells). Thus, a plurality of base stations can simultaneously transmit signals intended for the corresponding terminal. For this reason, since the terminal can handle signals from other cells not as interference signals but as desired signals, the SINR observed at the terminal can be expected to improve. Furthermore, improving a method of generating precoding weights in a plurality of cells as an operation within a network allows a large performance improvement to be achieved.
For such CoMP control, there is a method of observing channel information between the terminal and each cell to be targets of CoMP control and reporting the information as channel information in cell units (transmission point units) to the network.
A transmission point from a viewpoint of observing such channel information may also be called “CSI-RS-Resource” as an entity that transmits the aforementioned CSI-RS.
In addition, there is a heterogeneous network using a plurality of base stations having coverage areas different in size. The heterogeneous network is a network in which a macro base station that covers a large coverage area (called a “macrocell,” “HPN (High Power Node)” or “Macro eNB” in some cases) and a pico base station that covers a small coverage area (called a “picocell” or “Low Power Node (LPN)” or “Pico-eNB” in some cases) are used together. A method has been discussed by which mobility control (handover) is easily realized using the signal of the physical layer by giving the same identification number (cell ID) as a macrocell to a picocell disposed in the coverage area of the macrocell in the heterogeneous network. For the operation of such a heterogeneous network, a method is under study whereby channel information (CSI) measured by an LTE-A compliant terminal (hereinafter, referred to as “LTE-A terminal”) using CSI-RS newly added for the LTE-A terminal is reported to the network to select optimal transmission and reception points according to the propagation state (for example, refer to FIG. 1 and NPL 1).
Moreover, applying CoMP in a heterogeneous network is also being discussed. For example, receiving quality in UE can be expected to improve by applying CoMP such as a CB scheme or JT scheme between LPN1 and Macro eNB shown in FIG. 1.